Use a typedef for signal and for coordinates (in MDEvents)

[mantid-builder]

Original Reporter: @jzikovsky

Original Owner: @jzikovsky

Rename CoordType to coord_t to match other typedef's like size_t, detid_t, etc.

Make a typedef for the signal type, signal_t, that will likely be a double.

That way, if we need to change the data size for these types of data (float for coordinates, double for signal, vice-versa?) it will be ~trivial. Especially useful if data sizes change in future and we want to fix it on all platforms.

[mantid-builder]

This issue was originally trac ticket 3025